The present invention relates to a device for detecting a flow of powder particles, particularly paint particles, said flow being arranged to flow in a transportation line between a reservoir holding powder particles and at least one powder spray gun of the kind wherein an electrical field charges the particles.
Electrostatic powder coating processes for application of paint-powder particles use spray guns of a kind wherein an electric field charges the paint powder particles, for instance guns known as corona guns. The powder particles are supplied to the gun from a powder reservoir by way of transportation lines. In industrial applications large dimension systems are often used wherein a plurality of guns and transportation lines, for instance in the number of 15, are arranged in parallel.
Problems arise when for some reason or other the paint powder particle flow is obstructed in the transportation line and the particles are prevented from reaching the gun. As a result, an insufficient amount of paint powder will reach the product to be coated.
When several guns are used simultaneously it is difficult to discern visually that a certain part of the product is coated to an insufficient degree with the powder for an acceptable layer of paint to be obtained in the subsequent heat treatment in an oven. Consequently, the fault is not corrected until the defect, i.e. the unsatisfactorily painted products, are discovered in the subsequent quality control step. Likewise, it is difficult to discern in which one of the guns that the obstruction occurs. For this reason it is often necessary to examine large parts of the transportation line system to find the fault. Fault-localizing and discarding are, however, time consuming and expensive.
The object of the present invention is to provide a simple and reliable device for detection of a flow of powder particles that may be used to monitor large transportation line systems.
This object is achieved in accordance with the present invention in a device of the kind defined in the introduction, which device is characterized in that a portion of the transportation line is provided on its interior face with an electrostatically friction-charging material, that a signal-carrying line interconnects a unit for measuring electrical signals and the electrostatically friction-charging material in said portion, and that said unit is provided with a third line to close the circuit between said unit and said material in said portion.
The electrostatically friction-charging material on the interior face of said transportation line portion is of such a nature that it is electrostatically charged by the friction arising upon contact between the powder particles of a flow streaming through the line and the material. The magnitude of the charge of the material is then measured by said unit for measuring electrical signals. In this manner the device in accordance with the invention provides a tool for detection of whether or not a particle flow is moving through the transportation line, since the measuring unit shows response only when particles flow through said portion of the transportation line.
The third line preferably is connected to earth.
In accordance with one embodiment of the invention all of said transportation line portion is manufactured from an electrostatically friction-charging material.
The powder flow can be generated by means of a pump that is connected to the transportation line.
Preferably, the electrostatically friction-charging portion of the line could be provided adjacent to the pump. It goes without saying that this portion of the line could be located upstream or downstream from the pump in the direction of travel of the powder flow. It could likewise be located inside the pump itself.
In large systems where several powder guns are arranged for simultaneous operation one detecting device in accordance with the invention may be arranged in each one of the transportation lines leading to the respective spray guns. The signals from each one of the measuring units of the respective detecting devices may be supplied to a control panel for system monitoring. Owing to this monitoring feature any flow obstruction in any one transportation line thus may be immediately detected and localized. Consequently, the fault may be remedied before it has led to production disturbances, and the fault-localizing time is reduced since the control panel will show in precisely which transportation line the fault is to be found.
In addition, the device in accordance with the invention is inexpensive and easy to install, in new as well as in existing transportation-line systems.